1. Field of the Present Invention
The invention relates to a chain drive device which is utilized as a power transmission to rotate one or more cam-shafts of an internal combustion engine such as an engine for a vehicle with a stated timing, which chain drive device comprising at least a plural sprockets and a chain having a plural links and wrapped around the sprockets, and which chain drive device transmitting a driving force between the sprockets through the chain. The invention especially relates to a chain drive device of the abovementioned type which is improved to reduce a vibrating force given from the chain to the sprockets.
2. Description of the Related Art
FIG. 10 shows a front view of a known chain drive device of the abovementioned type. This chain drive device 10 is explained below as, for example a device which rotates a cam-shaft for reciprocating inlet valves and exhort valves of an engine for a vehicle.
The chain drive device 10 consists of a crank-side sprocket 1 (with N teeth) directly connected to a crank-shaft (not shown) of the engine, a cam-side sprocket 3 (with 2N teeth) directly connected to a cam-shaft (not shown) of the engine, a chain 2 having a plural links 2a and wrapped around both of the crank-side sprocket 1 and the cam-side sprocket 3, and two chain guides 4, 5 which are provided along the chain paths between the crank-side sprocket 1 and the cam-side sprocket 3 and which regulate the path of the chain 2, prevent rattling of the chain 2 and give a tension to the chain 2. Thus, a driving rotating-force given from the crank-shaft to the crank-side sprocket 1 is transmitted from the crank-side sprocket 1 to the cam-side sprocket 3 through the chain 2, and further transmitted from the cam-side sprocket 3 to the cam-shaft.
In the chain drive device 10 in FIG. 10, the crank-side sprocket 1 engages with the chain 2 at an engaging point A and separates from the chain 2 at a separating point B. On the other hand, the cam-side sprocket 3 engages with the chain 2 at another engaging point A' and separates from the chain 2 at another separating point B'. The chain 2 is wrapped over the crank-side sprocket 1 with a wrapping angle .beta., and is wrapped over the cam-side sprocket 3 with another wrapping angle .beta.'. The crank-shaft and hence the crank-side sprocket 1 is rotating at an angular speed of .omega..
During a turn of the crank-side sprocket 1, N links 2a of the chain 2 pass through each of the engaging point A and the separating point B causing a cyclic movement which is called as a first order component of a chain engagement or a Nth order component of an engine rotation, therefore, due to changes of the path of the chain 2 from a straight path to a polygonal path at the engaging point A and from a polygonal path to a straight path at the separating point B, the links 2a repeat a forced displacement of magnitude .epsilon. as shown in FIG. 11, which forced displacement changes with time as shown in FIG. 12.
Thus, due to a force of inertia of the chain 2, a vibrating force having a cycle of N times of the angular speed .omega. i.e. sin (N.multidot..omega.) is given from the chain 2 to the crank-side sprocket 1 at each of the engaging point A and the separating point B. Similarly, such a vibrating force is given from the chain 2 to the cam-side sprocket 3 at each of the engaging point A' and the separating point B'.
These vibrating forces are transmitted to the crank-shaft and cam-shaft and vibrate a cylinder block etc. of the engine, so that these vibrating forces are emitted out from the engine causing aggravation of a noise level of the engine.
Such a phenomenon is conspicuous in a chain drive system used in an internal combustion engine, and is not become a problem in other common chain drive system. Because, in an internal combustion engine, driving speed of the chain is very high relatively to other common cases, so that the force of inertia of the chain increases in proportion as the chain speed raised to the second power, thus the vibrating forces produced by the force of inertia tend to become especially large and become conspicuous. That is, the abovementioned phenomenon is a problem almost peculiar to a chain drive system used in an internal combustion engine.
A prior art chain drive device aiming to solve the problem is disclosed in the Japanese Patent Application Laid-open No. 193959/58. In this chain drive device, the forced displacement is restrained by providing a roller which pushes the chain path in by force in a distance corresponding to the magnitude .epsilon. of the forced displacement just before the chain is engaged with the sprocket, so as to reduce a striking noise produced by a collision between the chain and the sprocket.
Though, it is found by a phenomenon analysis carried by inventors of the present invention that, especially in a low rotating speed region (e.g. a rotating speed region below 2500 rpm), the noise emitted due to the vibrating forces is dominant rather than the striking noise produced by the collision.
Therefore, the prior art considering only the chain engaging point cannot achieve a sufficient noise reduction effect. Supposing that the prior art chain drive device is further provided a roller for restraining the forced displacement at a chain separating point so as to achieve a sufficient noise reduction effect, the chain drive device is required to provide such a roller for each of the engaging point A, A' and the separating point B, B' other than the chain guides 4, 5 as shown in FIG. 10, which chain drive device construction, as a matter of fact, invites other problems such as an enlargement of an engine size etc. except for there being a sufficient space, and which construction invites further problems of e.g. an occurrence of a new kind of vibration at a contact point between the roller and the chain.